Conventionally, middle distillate range products, e.g. heavy naphtha, kerosene, jet fuel, diesel oil and light cycle oil (LCO), are produced in petroleum refineries by atmospheric/vacuum distillation of petroleum crude and also by the secondary processing of vacuum gas oil and residues or mixtures thereof. Most commonly practiced commercial secondary processes are Fluid Catalytic Cracking (FCC) and Hydrocracking. Hydrocracking employs porous acidic catalysts similar to those used in catalytic cracking but associated with hydrogenation components such as metals of Groups VI and VII of the Periodic Table to produce good quality of middle distillate products in the boiling range of C8–C24 hydrocarbons. An excess of hydrogen is supplied to the hydrocracking reactor under very high pressure (150–200 atm.) and at a relatively lower temperature (375–425° C.) in fixed bed reactors with two phase flow. Due to severe hydrogenation, all hydrocarbon products from Hydrocracker are highly saturated with low sulfur and aromaticity. The yield of middle distillate hydrocarbons (126–391° C. boiling range) in hydrocracking is typically very high, i.e. up to 65–80 wt % of feed.
FCC process, on the other hand, is employed for producing substantial quantities of high octane gasoline and LPG. In countries where demand for middle distillate product is higher, heavy cracked naphtha (HCN: C8–C12 hydrocarbons) and light cycle oil (LCO: C13–C24 hydrocarbons) production are maximized by manipulating operating variables so as to vary the reaction and regenerator severity levels. U.S. Pat. Nos. 3,894,931 and 3,894,933 address such operations. Typically, diesel yield in FCC is maximized by maintaining a lower reaction and regeneration severity and recycling of unconverted residual products. Catalysts with lower zeolite/matrix ratio and MAT (Micro Activity Test) activity of about 60–70 is normally preferred. By proper selection of FCC variables and innovations involving catalyst type and recycle of heavy cycle oil and residual slurry oil, distillate yield can be increased by considerable amount at the expense of Gasoline yield. As the FCC unit operation is shifted from gasoline mode to middle distillate maximization mode, the LCO cetane number increases and thus could be more useful for blending into the diesel pool.
However, while running at low severity operations for maximizing diesel yield, the unconverted bottom yield also increases to a significant extent and sometimes may even exceed 20 wt % of fresh feed, as against 5–6 wt % for the usual gasoline mode operation. The other drawback of low severity operation is in the high amount of recycle oil being fed to the bottom of the riser with fresh feed for further cracking. Firstly, this reduces the throughput of riser reactor and secondly, with a single riser and product fractionator, the recycle is nonselective. Consequently, diesel yield from FCC with a conventional cracking catalyst could be maximized up to 40–45 wt % in spite of running at low reaction severity (495° C. riser temperature) and fairly high recycle ratio (30% of fresh feed).
Besides the operation of conventional FCC in middle distillate maximization mode, there are several other processes aiming for improvement in middle distillate yield. U.S. Pat. No. 5,098,554 discloses a process of fluid catalytic cracking with multiple feed injection points where fresh feed is charged to upper injection points and unconverted slurry oil is recycled to a location below the fresh feed nozzles. Essentially, the process conditions are similar to that of gasoline mode FCC operation (e.g., 527° C. riser top temperature) which favors gasoline production. By adopting split feed injection, middle distillate yield is marginally increased at the expense of gasoline yield.
U.S. Pat. No. 4,481,104 describes about an ultra-stable Y-zeolite, of high framework silica to alumina ratio having low acidity and large pores, use of which in catalytic cracking of gas oil, enhances distillate yield. It may be noted that yield of 420–650° F. fraction is maximize about 28 wt % of feed and, as 650° F.− conversion increases beyond 67 wt %, the yield of 420–650° F. fraction reduces. Therefore, as discussed earlier, yield of the distillate is relatively higher only at the higher yield of unconverted fraction.
Yet another process, reported in U.S. Pat. No. 4,606,810, discloses a scheme of two riser cracking for improving total gasoline plus distillate yield. Here, the feed is first cracked in the first riser with spent catalyst from the second riser and the unconverted part is further cracked in a second riser in presence of regenerated catalyst. The basic operation is of high severity producing maximum amount of gasoline and the yield of LFO is around 15–20 wt % of feed. It may also be noted that while increase in gasoline yield is in the range of 7.5–8.0 wt %, increase in LFO yield is merely in the range of 1.5–3.0 wt % on fresh feed basis.
Two stage processing of hydrocarbon feedstock has been employed by different researchers in the field of catalytic cracking. Several processes have been developed in which first stage processing removes metals and Conradson Carbon Residue (CCR) impurities from feed using a low activity cheap contact material with abundant surface area. The demetallized feed is then processed in a more conventional second stage reactor under high severity conditions to maximize the conversion and gasoline production. U.S. Pat. No. 4,436,613 describes such a process of two stage catalytic cracking using two different types of catalyst. In the first stage, the CCR materials and metals are separated from the rest of the feedstock along with mild cracking over a relatively lower activity catalyst. The residual un-cracked product of the first stage is then contacted with a high activity catalyst under higher reaction severity for gasoline maximization. It may be noted that in this process, two dedicated strippers and regenerators are used to avoid the mixing of two different types of catalysts.
Dual riser high severity catalytic cracking process described in U.S. Pat. No. 3,928,172 utilizes a mixture of large pore REY zeolite catalyst and a shape selective zeolite catalyst where gas oil is cracked in the first riser in the presence of the aforesaid catalyst mixture. The heavy naphtha product from the first riser and/or virgin straight run naphtha are cracked in the second riser in the presence of catalyst mixture to produce high octane gasoline together with C3 and C4 olefins. U.S. Pat. No. 4,830,728 discloses a process for upgrading straight run naphtha, catalytically cracked naphtha and mixtures thereof in a multiple fluid catalytic cracking operation utilizing mixture of amorphous cracking catalyst and/or large pore Y-zeolite based catalyst and shape selective ZSM-5 zeolite catalyst to produce high octane gasoline.
U.S. Pat. No. 5,401,387 describes a process of multistage catalytic cracking where the first stage cracks a first feed over a shape selective zeolite to produce lighter products rich in iso-compounds which may be used for making ethers. A second feed, which may include 700° F.+ liquid from first stage, is cracked in the second stage. Another process, as described in U.S. Pat. No. 5,824,208, discloses a scheme in which hydrocarbon is initially contacted with cracking catalyst forming a first cracked product which, after recovering of the product fraction having boiling point of more than 430° F., is subjected to cracking in a second riser. The basic objective of this invention is to maximize the yield of light olefins and minimize the formation of aromatic compounds by avoiding undesirable hydrogen transfer reactions.
So far, most of the prior art methods have concentrated on multiple riser catalytic cracking for maximization of gasoline yield and its octane numbers, increased yield of iso-olefin for production of ethers, increased yield of light olefins, etc. From the prior art information and also from our experience of operating low severity FCC units, it is quite clear that maximizing middle distillate yield in FCC (without using external hydrogen) is not achieved beyond a level of 40–45 wt % of fresh feed. Further, persons involved in fluid catalytic cracking would be aware that middle distillate, being an intermediate product in the complex catalytic cracking reactions, is very difficult to maximize because, when the severity is increased, it is over cracked to lighter hydrocarbons.
Objects
Accordingly, one object of the present invention aims to propose a novel catalytic cracking process for producing middle distillate products in very high yield (about 50–65 wt %).
Another object is to provide a multiple riser system that enables the production of middle distillate products, including heavy naphtha and light cycle oil in high yield.
Yet another object of the invention is to provide a multiple riser system to produce higher yield of heavy naphtha and light Cycle Oil as compared to the prior art processes employing catalytic cracking of petroleum feedstock without any use of external supply of hydrogen.
A further objective of the process is to minimize the yield of unwanted dry gas and coke and also the yield of unconverted bottom products, at the same time, improving the cetane quality of the middle distillate product.